Liquid crystal display devices are applied to, for example, watches, calculators, a variety of household electrical appliances, measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, and television sets. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, a DS (dynamic scattering) type, a GH (guest-host) type, an IPS (in-plane switching) type, an OCB (optically compensated birefringence) type, an ECB (electrically controlled birefringence) type, a VA (vertical alignment) type, a CSH (color super homeotropic) type, and an FLC (ferroelectric liquid crystal) type. Examples of a drive system include static driving, multiplex driving, a passive matrix, and an active matrix (AM) in which, for example, a TFT (thin film transistor) or a TFD (thin film diode) is used for driving.
Among these types of liquid crystal display devices, an IPS type, an ECB type, a VA type, and a CSH type are characterized in that a liquid crystal material having a negative Δε is used. In particular, VA display devices of AM driving are applied to display devices that need to quickly respond and to have a wide viewing angle, such as television sets.
Nematic liquid crystal compositions used in, for instance, VA display devices need to enable driving at low voltage, a quick response, and a broad range of operating temperature. In other words, a liquid crystal composition having a negative Δε with a large absolute value, low viscosity, and a high nematic phase-isotropic liquid phase transition temperature (Tni) has been demanded. In order to control Δn×d that is the product of refractive index anisotropy (Δn) and a cell gap (d) to be a predetermined value, the Δn of a liquid crystal material needs to be adjusted to be in a proper range on the basis of the cell gap. In addition, a quick response is important in liquid crystal display devices applied to television sets or other apparatuses, which generates a need for a liquid crystal material having a small viscosity (η).
A variety of compounds having a negative Δε with a large absolute value have been studied to improve the properties of liquid crystal compositions.
A liquid crystal composition containing the following liquid crystal compounds (A) and (B) each having a 2,3-difluorophenylene structure has been disclosed as a liquid crystal material having a negative Δε (see Patent Literature 1).

This liquid crystal composition also contains liquid crystal compounds (C) and (D) as compounds having a Δε of substantially zero; however, the liquid crystal composition does not have a sufficiently low viscosity that is necessary in applications in which a quick response is needed, such as liquid crystal television sets.

A liquid crystal composition containing a compound represented by Formula (E) has been disclosed; however, this composition is a liquid crystal composition in which the liquid crystal compound (D) is used in combination and which has a small Δn (see Patent Literature 2) or a liquid crystal composition which contains a compound of which the molecules each contain an alkenyl group (alkenyl compound), such as a liquid crystal compound (F), to improve response speed (see Patent Literature 3). Further study has been necessary in order to give both high Δn and high reliability.

A liquid crystal composition containing a compound represented by Formula (G) has been disclosed (see Patent Literature 4), but this composition is also a liquid crystal composition containing an alkenyl compound such as the liquid crystal compound (F). Thus, the liquid crystal composition has a problem in which defective display such as image-sticking and uneven display is likely to be caused.

The effect of the liquid crystal composition containing an alkenyl compound on defective display has been disclosed (see Patent Literature 5). In general, a decrease in the alkenyl compound content leads to an increase in the η of a liquid crystal composition, and sufficiently quick response speed is therefore not produced; thus, both requirements of reduced defective display and quick response have not been satisfied at the same time.
Merely combining a compound having a negative Δε with the liquid crystal compound (C), (D), or (F) as described above is not enough to produce a liquid crystal composition that has not only a negative Δε but also both high Δn and low η and that enables a reduction or elimination of defective display.
Moreover, another technique has been disclosed in Patent Literature 6, in which a liquid crystal material having a large index (FoM) represented by (Equation 1) is used to enhance the response speed of a homeotropic liquid crystal cell; however, the enhancement in the response speed of the liquid crystal composition, which has been explained in Description thereof, is insufficient.
[Math. 1]FoM=K33·Δn2/γ1  (Equation 1)
K33: Elastic constant
Δn: Refractive index anisotropy
γ1: Rotational viscosity
From such viewpoints, a liquid crystal composition that enables a quick response speed that is necessary in applications such as liquid crystal television sets needs to have both the quality (reliability) that defective display such as image-sticking and uneven display is less likely to be caused and the following properties without reductions in refractive index anisotropy (Δn) and nematic phase-isotropic liquid phase transition temperature (Tni): sufficiently low solid phase-nematic phase transition temperature (Tcn), sufficiently small viscosity (η), sufficiently small rotational viscosity (γ1), and a large elastic constant (K33).